1. Field of the Invention
The present invention relates to an optical polarization controller employed in optical heterodyne detecting devices which are used for optical communication, optical information processing or optical measurement.
2. Background Art
Optical heterodyne detecting devices are used in optical communication systems as demodulation means. In the communication system, a signal light, which is modulated based on information to be sent, is transmitted from a transmitter, and the signal light is received by a receiver. In the receiver, a local oscillation light is generated by a local oscillator of the heterodyne detecting device and the local oscillation light is supplied to a mixer. The received signal light is mixed with the local oscillation light by the mixer. As a result, a beat signal indicating the information sent from the transmitter is obtained.
In the optical heterodyne detecting devices, when mixing the input signal light and the local oscillation light, the plane of polarization of the input signal light should correspond to the plane of polarization of the local oscillation light.
If the planes of polarization of the input signal light and the planes of polarization of the local oscillation light cross each other, the sensitivity of the receiving light signal is degraded. Specifically, if the plane of polarization of the input signal light is perpendicular to the plane of polarization of the local oscillation light, there is no interference between the lights and no beat signal is obtained from the mixer. Therefore, no information can be detected from the input signal light.
In order to overcome this problem, a polarization control element is provided in the optical heterodyne detecting device.
FIG. 4 shows the configuration of a conventional device. In FIG. 4, a polarization control element 2 which receives an input signal light 2A to be demodulated. The polarization control element rotates the plane of polarization of the signal light 2A and outputs a signal light 2B which has a rotated plane of polarization. The rotation angle between the planes of polarization of signal light 2A and of polarization of signal light 2B is controllable.
A light mixer 31 mixes the signal light 2B and a local oscillation light 7. The light mixer 31 outputs a pair of balanced signal lights which indicate the mixed result. The balanced signal lights complement each other. That is to say, the balanced signal lights have the same amplitude, and the phase of one of the balanced signal lights is shifted by 1/2 period from the phase of the other.
A balanced type-receiver 32 has two photo-detectors and an electronic differential amplifier (not shown). The balanced signal lights, which are outputted by the light mixer 31, are converted into electronic signals by the photo-detectors. The electronic signals thus converted are inputted to the differential amplifier and differential amplification is carried out. As a result, a beat signal 32A having a frequency corresponding to the frequency difference between the signal light 2B and the local oscillation light 7 is obtained from the differential amplifier. In this differential amplification, common mode noises of the two electronic signals are canceled. Therefore, in the beat signal thus obtained, the noise component due to the DC level variations of the balanced signal lights is reduced. Furthermore, the noise components due to the variation in time of the light amount of the input signal light 2A and to AM (amplitude modulated) noise of the local oscillation light 7 are also reduced.
In this configuration, if the rotation angle of the plane of polarization of the polarization control element 2 is gradually varied, the output level of the beat signal 32A becomes maximum when the plane of polarization of signal light 2B corresponds to the plane of polarization of local oscillation light 7.
Therefore, it is possible to optimize the plane of the signal light 2B by monitoring the level of the beat signal 32A and by controlling the rotation angle of the polarization rotating component 2 so that the beat signal 32A having the maximum level is monitored.
However, the background art has the following problems:
(1) When the frequency difference between the signal light 2A and the local oscillation light 7 is greater than the pass band width of the balanced-type receiver 32, no beat signal is obtained from the balanced-type amplifier. PA1 (2) When the plane of polarization of the signal light 2B is perpendicular to the plane of polarization of the local oscillation light 7, no beat signal is obtained.
On the other hand, no beat signal is obtained when the frequency difference between the lights 2A and 7 is greater than the pass bandwidth as described above.
Therefore, if no beat signal is monitored, it is impossible to judge whether the frequency difference is greater than the pass bandwidth or the planes of polarization are perpendicular to each other.